Multilayer thin-film structures deposited upon a substrate are widely used in optical applications, such as in Fabry-Perot filters, and in microelectronic applications. The layers may be metals or inorganic nonmetals. In one approach to fabricating multilayer thin-film structures, several evaporation sources are placed into a vacuum chamber, with each evaporation source in a line of sight with a substrate. A plurality of thin layers of different compositions are deposited serially onto the substrate from the various evaporation sources.
Evaporatively unstable materials are particularly difficult to evaporate and deposit onto the substrate in a controlled manner. Such materials are unstable because, for example, they exhibit a rapid and/or nonlinear change in evaporation rate with power input, they sublime, or they have a small difference between the melting and boiling temperatures. Many important evaporatively unstable materials are inorganic nonmetals, such as glasses, ceramics, or semiconductors.
One common approach to depositing evaporatively unstable materials is to use thermal sources, wherein the evaporatively unstable material is placed into a crucible that is heated to a sufficiently high temperature to cause the evaporatively unstable material to evaporate and travel to the substrate. Such thermal sources are operable and highly useful in depositing thick films, but they are difficult to use in the transient conditions of thin-film deposition and with many evaporatively unstable materials.
An alternative is to use electron-beam evaporation. In an electron-beam deposition system, an evaporation source includes a deposition material and a controllable electron-beam source that directs an electron beam at the exposed surface of the deposition material. The electron beam heats the surface of the deposition material so that it progressively evaporates and is deposited upon the substrate. With time, the thickness of the deposit increases to a desired value. For thin-film deposition, electron-beam deposition has the advantage that the evaporation can be started and stopped rapidly through control of the beam power and location.
However, electron-beam deposition may be difficult to control, particularly for the deposition of evaporatively unstable materials such as some inorganic nonmetallic materials. The usual control approach is to continuously monitor the thickness of the film deposited upon a monitored substrate different from the substrate of the article of manufacture. The change in thickness is used to control the power input to the electron-beam source that determines the subsequent evaporation rate. Although successfully used for metallic materials, this approach has drawbacks for the deposition of evaporatively unstable materials. Such materials exhibit instabilities in the deposition process and the deposition rate, so that control based upon thickness measurements of the deposit on the monitored substrate may result in incorrect thicknesses on the article of manufacture.
There is a need for a better approach to the deposition of evaporatively unstable materials to produce thin-film structures. The present invention fulfills this need, and further provides related advantages.